The subject matter herein relates generally to semiconductor microelectromechanical (MEMS) based sensor configurations that can be used to detect small forces or flexures generated from mechanical stress, chemo-mechanical stress, thermal stress, electromagnetic fields, and the like. More particularly, the subject matter disclosed herein relates to a MEMS based pressure sensor and a method for fabricating the same.
Advances in semiconductor microelectronic and MEMS based sensors have served greatly to reduce the size and cost of such sensors. The electrical and mechanical properties of silicon microsensors have been well chronicled. Silicon micromachining and semiconductor microelectronic technologies have blossomed into a vital sensor industry with numerous practical applications. For instance, micromachined silicon pressure sensors, acceleration sensors, flow sensors, humidity sensors, microphones, mechanical oscillators, optical and RF switches and attenuators, microvalves, ink jet print heads, atomic force microscopy tips and the like are widely known to have found their way into various applications in high volume medical, aerospace, industrial and automotive markets. The high strength, elasticity, and resilience of silicon makes it an ideal base material for resonant structures that may, for example, be useful for electronic frequency control or sensor structures. Even consumer items such as watches, scuba diving equipment and hand-held tire pressure gauges may incorporate silicon micromachined sensors.
The demand for silicon sensors in ever expanding fields of use continues to fuel a need for new and different silicon microsensor geometries and configurations optimized for particular environments and applications. Unfortunately, a drawback of traditional bulk silicon micromachining techniques has been that the contours and geometries of the resulting silicon microstructures have been significantly limited by the fabrication methods. For instance, etching silicon structures with conventional etching techniques is constrained, in part, by the crystal orientations of silicon substrates, which limits the geometry and miniaturization efforts of many desired structures.
The increasing use of microsensors to measure pressure has spurred the development of small silicon plate structures used, for example, as capacitors and to produce electrostatic forces. For instance, there exist microsensors that measure capacitance using an array of interdigitated polysilicon plates. Similarly, there exist microsensors that produce electrostatic forces using an array of layered plates. Further, there exist microsensors that measure the flexure, or bending, of silicon structures in response to forces such as pressure or acceleration.
Measurements of biological parameters using microsensors are becoming increasingly common and important for both diagnostic and patient monitoring purposes. In some applications, in-vivo catheter tip pressure sensors are used to measure either absolute pressure or differential pressure based on a given reference pressure, such as atmospheric pressure. For example, differential catheter tip pressure sensors can be used to measure the breathing of a human being based on pressure changes within the respiratory system with respect to atmospheric pressure. The expanding fields of use of microelectromechanical devices in general, and of catheter tip pressure sensors in particular, has created a demand for ever smaller devices. Unfortunately, there has been difficulty producing smaller devices that are also highly sensitive to small changes in pressure which can be effectively manufactured in high volumes.
Sensors manufactured through conventional fabrication techniques are limited with respect to their size and packaging. For example, the elongated nature of a catheter tip pressure sensor requires that electrical connections extend from one end of the sensor, typically the end that is not inserted, to the sensing portion of the device. These connections can detrimentally impact the size and shape of the resulting device. Additionally, because of the small size of the devices and the thin nature of the geometries used, conventional techniques for producing such micromechanical devices risk both breakage during the manufacturing process and potentially diminished reliability in the field. For example, since differential catheter tip pressure sensors measure pressure relative to a reference pressure, a vent from the sensor to an external reference pressure must be supplied. This is typically done through a fine capillary tube that is run to the catheter tip in parallel with the electrical connections along the back of the chip. However, this configuration can result in thicker packaging of the sensor and can result in the vent becoming pinched-off during measurement. Other fabrication techniques employ side vent configurations that exit the chip through vent ports located on one of the chip edges, but which require additional processing steps to create the vent port, such as sawing, that can result in entry of debris into the vent port and diminish both accuracy and reliability.
It would be advantageous to provide a method for manufacturing highly sensitive pressure sensors that are not only small in size, but which can be effectively produced in high volume.